1. Field of the Invention
The present invention relates to a method of manufacturing a hexagonal ferrite magnetic powder, and more particularly, to a method of manufacturing a hexagonal ferrite magnetic powder that is suitable as the magnetic powder of a magnetic recording medium employed for recording and reproduction at surface recording densities of 1 Gbpsi and above.
The present invention further relates to a magnetic recording medium employing the hexagonal ferrite magnetic powder obtained by the above manufacturing method, and to a method of manufacturing the same.
2. Discussion of the Background
Recently, ferromagnetic metal powders have come to be primarily employed in the magnetic layers of magnetic recording media for high-density recording. Ferromagnetic metal powders are comprised of acicular particles of mainly iron, and are employed in magnetic recording media for various applications in which minute particle size and high coercivity are required for high-density recording.
With the increase in the quantity of information being recorded, magnetic recording media are required to achieve ever higher recording densities. However, in improving the ferromagnetic metal powder to achieve higher density recording, limits have begun to appear. By contrast, hexagonal ferrite magnetic powders have a coercivity that is high enough for use in permanently magnetic materials. Magnetic anisotropy, which is the basis of coercivity, derives from a crystalline structure. Thus, high coercivity can be maintained even when the particle size is reduced. Further, magnetic recording media employing hexagonal ferrite magnetic powder in the magnetic layers thereof can afford good high-density characteristics due to their vertical components. Thus, hexagonal ferrite magnetic powder is an optimal ferromagnetic material for achieving high density.
For example, Document 1 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 7-201547), Document 2 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-87002), Document 3 (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-51490) or English language family member U.S. Pat. No. 4,582,623, Document 4 (Japanese Unexamined Patent Publication (KOKAI) No. 2006-5299) or English language family member US 2005/0282043 A1, Document 5 (Japanese Unexamined Patent Publication (KOKAI) No. 2006-5300) or English language family members US 2007/0020489 and U.S. Pat. No. 7,381,482, and Document 6 (Japanese Unexamined Patent Publication (KOKAI) No. 2006-41493) or English language family members US 2005/0282042 A1, U.S. Pat. No. 7,132,164 and US 2006/0051624 A1 propose manufacturing the hexagonal ferrite magnetic powders used in magnetic recording by the glass crystallization method. The contents of the above applications are expressly incorporated herein by reference in their entirety. In addition to the glass crystallization method as a method of manufacturing hexagonal ferrite powder, other known methods include the water heating synthesis method and the coprecipitation method. However, the glass crystallization method is superior as a method of manufacturing hexagonal ferrite for magnetic recording media, from the perspectives of suitability of the microparticles, suitability of the dispersion of single particles, sharp particle size distribution, and the like, which are desirable for use in magnetic recording media.
Document 1 proposes maintaining the magnetization level of the amorphous material to 0.1 to 2 emu/g to obtain hexagonal ferrite with a good SFDr. Documents 2 to 6 propose achieving uniform microparticles and grain size distribution through the starting material composition of the hexagonal ferrite to obtain hexagonal ferrite that is suited to high-density recording.
The density of recording has continued to increase in recent years. Recording densities of equal to or greater than 1 Gbpsi are now being targeted. Under such conditions, it has become difficult to provide hexagonal ferrite magnetic powders capable of achieving the targeted recording densities with glass crystallization methods, including the methods described in the above applications.